Over the past ten years the field of spinal surgery has undergone significant growth. From 1997 to 2003, spinal fusions have risen in ranking from among the 41 most common inpatient procedures to among the 19 most common. During this same period, image guided surgery (IGS) also has undergone significant growth. At the University of Florida (UF) Shands Teaching Hospital, in this same period, the number of spine related image guided surgical cases has increased from 25 per year to over 750 per year. Moreover, the increased use of IGS in spine surgery is not simply a function of the increased number of total surgical procedures performed. UF's 30-fold increase in IGS utilization is due primarily to the increased accuracy and flexibility provided by commercially available image guidance platforms.
In the late 1990s, the state of the art for image guidance in spine surgery was based generally on one of four paradigms: 1) Live fluoroscopic guidance, 2) Pre-surgical computed tomography (CT) scans, 3) Virtual fluoroscopy, and 4) Two-dimensional to three-dimensional (2D-3D) Registration. Live fluoroscopic guidance remains the most common intraoperative guidance approach. Spine surgeons routinely rely upon antero-posterior (AP) views to localize their entry point and lateral fluoroscopy to refine their trajectories. While effective, this technique can require the surgeon to be immediately adjacent to either the X-ray tube or image sensor, resulting in significant exposure. Recent measurements in our laboratory have equated the level of thyroid exposure from a typical three-level spine fusion to approximately 30 chest X-rays.
Pre-surgical CT scans provide high-resolution reformatted views through the surgical region of interest. While providing the best intraoperative images, the virtual image created by these scans must be registered to the patient intraoperatively. In this context, the term “register” refers to mapping reference points on the image to corresponding points on the patient's anatomy. This image-to-patient registration process is more problematic and more time consuming when applied to spine surgery than when used for cranial guidance i.e., the area where the technique was first pioneered. There is little bone available in a small volume for registration, and it is difficult to identify corresponding points in the operative field and the preoperative CT based virtual model. These technical challenges result in inaccurate registrations, and at times it is not possible to derive an acceptable model-to-patient transformation.
Virtual fluoroscopy is an attempt to overcome the drawbacks of live fluoroscopy based guidance. In this technique, one first acquires fluoroscopic views at known geometries, usually AP and lateral views. The C-arm orientation is recorded at the time of acquisition and linked to the images, permitting creation of virtual fluoroscopic views. The system then actively tracks instruments and superimposes them in a virtual fluoroscopic view. This allows simultaneous guidance on both AP and lateral images and does not require the surgeon to be in the imaging field during X-ray exposures. Superficially, this technology appears to provide the surgeon with a real advantage in personal dose reduction. Yet this technology has not gained popularity because it apparently lacks any technological advantage, does not provide unique or special views, generally requires complex image acquisition hardware, and typically requires the surgeon to hold the instrument whenever live fluoroscopic validation is required.
2D-3D Registration uses two orthogonal fluoroscopic views to register a pre-surgical CT scan. This is an attempt to provide the high quality reformatted images from pre-surgical CT scan with an easier, less invasive fluoroscopic registration process. In practice, it is often difficult to provide unambiguous planar views as required for accurate 2D-3D matching, making accurate registration problematic.
Despite all of the previously discussed drawbacks, each of these four guidance technologies provided a three-fold reduction in the rate of screws breaching the pedicle wall during placement of pedicle screws. Although this represents a significant increase in the accuracy of screw placement, the difficulties associated with system operation have severely limited system acceptance of these four guidance technologies.
Over the past three years the introduction of a fifth technology has reenergized image guided spine surgery. Intraoperative Cone-Beam CT (CB-CT) based spine IGS has provided a significant advance in overall system utility. The automatic registration of intraoperative cone beam images has provided good quality orthogonal views of the surgical trajectories while simultaneously eliminating the largest source of inaccuracy in applying pre-surgical CT images—i.e., the model-patient registration process.
These increasingly capable guidance technologies have increased the accuracy and precision of spine surgery, but they have not decreased operative time, provided a system (including the imaging chain) which can be operated from within the surgical field, or provided intraoperative image quality comparable with diagnostic CT scanners. The complexity of the guidance system and the complicated choreography of draping the imaging chain have provided special challenges to operating room staff (see, e.g., FIGS. 1a-1c). The issues associated with surgeon access in the surgical field after the images have been obtained have also impeded wider acceptance of existing systems (see, e.g., FIG. 1b). It is also common practice for each spine instrument vendor to design unique instruments to accompany each new screw, plating, or rod system. This translates into a need for each IGS manufacturer to engineer unique mechanical adapters for each new drill, tap, and screwdriver. These unresolved technical and practical problems continue to slow acceptance of image guidance in spine surgery.